1. Field of the Invention
The present invention relates generally to the field of computer systems. More particularly, the present invention relates to the field of bus termination schemes.
2. Description of Related Art
Transmitting electrical signals over relatively long mismatched bus transmission lines between integrated circuit devices typically creates signal reflections. Signal reflections occur when only a portion of the power of a signal transmitted over a transmission line is output to the load at the end of the transmission line while the remaining portion of the power of the signal is reflected back onto the transmission line. The existence of signal reflections on transmission lines can result in overshoots, undershoots, and/or ringbacks, for example.
An overshoot typically occurs when the voltage of a received signal rises from a lower value, such as 0 Volts (V) for example, beyond a desired higher value, such as 3.3 Volts (V) for example, before settling near the higher value. An undershoot typically occurs when the voltage of a received signal falls from the higher value beyond the lower value before settling near the lower value. Ringbacks typically occur when a signal overshoot falls back below the higher value before settling near the higher value or when a signal undershoot rises back above the lower value before settling near the lower value.
Overshoots, undershoots, and ringbacks increase the amount of time required to read signals because the circuitry receiving each signal must wait relatively longer for the signal to settle near the higher or lower value prior to interpreting the signal as a logical one or zero, respectively. Because signals may be transmitted only as fast as they can be read, overshoots, undershoots, and ringbacks limit the frequency at which signals may be transmitted over a transmission line. Additionally, relatively large overshoots can damage circuitry designed to receive signals at voltage values of only some predetermined amount over the higher value.
To avoid or minimize signal reflections and therefore help maintain signal quality to allow signal transmission at relatively high frequencies, each transmission line may be terminated with a load having an impedance that is approximately equal to the characteristic impedance of the transmission line.
FIG. 1 illustrates a block diagram of a computer system 100 comprising a motherboard 102, a first processor card 110 supporting a first processor 112, a second processor card 120 supporting a second processor 122, and a chipset 130. Motherboard 102 has a processor bus formed by a plurality of transmission lines on motherboard 102. One exemplary transmission line 104 is illustrated in FIG. 1. Processor cards 110 and 120 are each mechanically and electrically coupled to the processor bus of motherboard 102 by insertion into a respective slot connector on motherboard 102. Chipset 130 is mechanically and electrically coupled to motherboard 102. At least one chip of chipset 130 is mechanically and electrically coupled to the processor bus of motherboard 102.
Processor 112 comprises on-die terminating pull-up resistors at or near the end of each transmission line at processor 112. Processor 122 comprises on-die terminating pull-up resistors at or near the end of each transmission line at processor 122. The terminating resistors each have an impedance approximately equal to the characteristic impedance of its corresponding transmission line to help avoid or minimize signal reflections at processors 112 and 122. As one example, processor 112 comprises an on-die terminating pull-up resistor 116 at or near its end of transmission line 104, and processor 122 comprises an on-die terminating resistor 126 at or near its end of transmission line 104.
The end of each transmission line at chipset 130 is not terminated with a resistor to help reduce design complexity and power dissipation that results from constant current flow through terminating resistors as a result of driving either end of each transmission line low. Because this single-ended termination scheme produces signal reflections at the end of each transmission line at chipset 130, the length of each stub for chipset 130, such as stub 134 for example, is relatively short to help minimize such signal reflections.
Computer system 100 may be changed to a uni-processor system by removing processor card 120, for example, from its slot connector on motherboard 102. Because the end of each transmission line at that slot connector would no longer have terminating resistors following removal of processor card 120, a bus termination card comprising terminating resistors for each such transmission line is typically inserted into the slot connector to avoid or minimize signal reflections. One or more resistors of a bus termination card, however, may not be firmly coupled to bus termination card and/or may become loose, for example, from rough handling by a user. As a result, one or more transmission lines may not be terminated by the bus termination card. A bus termination card also incurs an added expense for computer system 100.